Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode on both faces of the membrane-electrolyte.
The MEA generally comprises porous conductive materials, also known as gas diffusion media, which form the anode and cathode electrode layers. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and hydrogen cations. The electrons are conducted from the anode to the cathode through an electrical circuit connected therebetween. Simultaneously, the hydrogen cations pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the hydrogen cations to form water as a reaction product.
The MEA is generally interposed between a pair of electrically conductive contact elements or separator plates to complete a single PEM fuel cell. Separator plates serve as current collectors for the anode and cathode, and have appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants (i.e., the H2 & O2/air) over the surfaces of the respective electrodes.
In practice, however, PEM fuel cells are not individually operated. Rather, PEM fuel cells are connected in series, or stacked one on top of the other, to form what is usually referred to as a fuel cell stack. PEM fuel cell stacks are generally loaded in compression in order to maintain low interfacial electrical contact resistance between the separator plates, the gas diffusion media, and the catalyst electrode. The low interfacial contact resistance in a PEM fuel cell stack is directly related to the compression loading. A typical compression load on the separator plate may range from about 50 to about 400 psi and is controlled by a compression retention system.
Compression retention systems are typically designed in a manner effective to offset strains produced by membrane swelling that can occur with changes in humidity and temperature and compressive stress relaxation in the fuel cell stack. Such systems act to minimize an over-compression and damage of gas diffusion media in the fuel cell stack, as well as maintain the desired stack compression and contact pressure between separator plates, gas diffusion media, and catalyst layers. Maintenance of stack compression sustains electrical contact and facilitates lower contact resistance between individual fuel cells within the stack.
PEM fuel cell stacks also generally include side plates for holding individual fuel cells laterally in place and generally for enclosing the fuel cell stack. Such systems are typically rigid and fixed in place, often requiring complex seals, plumbing and electrical connections to account for the swelling and contraction (also known as breathing) of the fuel cell stack that occurs with humidity and temperature cycling during operation.
It is stated in U.S. Pat. No. 5,484,666 to Gibb et al. that conventional compression systems have consisted of tie rods extending through and between endplate assemblies and secured with fastening nuts. Springs threaded on the tie rods and interposed between the fastening nuts and the endplates have been used to apply resilient compressive force to fuel cell stacks in the stacking direction.
Fuel cell side plates having controlled tensile compliance are also reported in U.S. Pat. Appl. Pub. No. 2006/0040166 to Budinski et al. It is stated in Budinski et al that the compression forces on a fuel cell stack can be controlled by incorporating at least one spring element into a side plate.
In U.S. Pat. No. 5,789,091, Wozniczka et al. further reports a mechanism for securing a fuel cell stack in an assembled and compressed state which includes at least one compression band that circumscribes end plate assemblies and interposed fuel cells in the fuel cell stack.
There is a continuing need for a compression retention system for electrochemical fuel cells that is compliant, provides substantially uniform compression during fuel cell stack operation and with a variety of stack sizes, facilitates relative movement between the retention system and fuel cell stack, and optimizes volumetric and thermal efficiency.